X-radiation (composed of X-rays) is a form of electromagnetic radiation. X-rays have a wavelength in the range of 10 to 0.01 nm, corresponding to frequencies in the range 30×1015 Hz to 30×1018 Hz, and energies in the range 120 eV to 120 keV. X-rays are used for medical radiography, crystallography, non-destructive testing, security etc. X-rays are in practice generated by an X-ray tube, in some cases by synchrotron radiation.
State of the art X-ray image sensing is performed with pixels that are either based on charge or current integration (or “integrating”) or photon counting (which are based on “pulse detection”).
X-ray detection can be “direct”, where an X-ray photon creates a packet of secondary electron-hole pairs in semiconductor material. The secondary electron-hole pairs are subsequently read by a charge sensitive amplifier. Alternatively, X-ray detection can be “indirect”, where an X-ray photon is absorbed in a scintillator material where it creates a flash of secondary, visible light, which secondary light is subsequently detected by a visible light image sensor.
In case of e.g. charge integration combined with indirect detection, the image sensor is used for detection of the secondary visible radiation. The image sensor converts this visible radiation into analog electrical signals (current, charge or voltage). The more radiation impinges on the image sensor integrated over time, the higher the corresponding electrical signal. Integrating pixels are simple and well-known electronic circuits. These have three or a few more transistors. Many examples exist in the state of the art, of which a very simple one is represented in FIG. 1. The pixel 10 illustrated comprises a radiation transducer such as a photodiode 11, for converting radiation impinging on the pixel 10 into an electrical signal such as for example a photocurrent 12. The thus generated photocurrent 12 integrates on an integrating element such as a capacitor 13. A voltage buffer 14 may be provided between the integrating element 13 and a multiplexing network 15 or read-out circuitry for reading out a radiation value.
X-radiation consists of separate X-ray photons, which can be detected as separate electrical pulses. A pulse detecting pixel, which is often, but not always, also a photon counting pixel, may be superior to an integrating pixel for signal to noise reasons. However, such counting pixels are complex devices. They have an analog front-end that detects pulses (the front end for example comprising a pulse shaper, a comparator) and an elaborate digital counter. The number of transistors is in the hundreds (100 . . . 1000).
It is generally understood that the pulse detecting or counting approach is superior over the integrating approach in terms of noise, especially at low radiation levels. The counting of photons is essentially noise free (apart from the inherent photon shot noise, PSN), whereas with integration, the collected noise charge is contaminated by analog “read noise”.
A further advantage of photon counting is that one can do at the same time energy (or wavelength) discrimination, and thus obtain at the same time images for different bands of photon energies (which is referred to as “color X-ray”, similar to the wavelength discrimination in visible light image sensors).
In the applicant's patent application US2010/213353, the burden of the huge transistor count of digital counting is somewhat relaxed by employing analog domain counting.
In “Counting and Integrating Readout for Direct Conversion X-ray Imaging—Concept, Realization and First Prototype Measurements”, Nuclear Science Symposium Conference Record, 2005 IEEE Volume 5, 23-29 Oct. 2005, pages 2761-2765, E. Kraft et al. report a signal processing concept for X-ray imaging with directly converting pixilated semiconductor sensors. At the basis of this approach lies the combination of charge integration and photon counting in every single pixel. Simultaneous operation of both signal processing chains extends the dynamic range of the pixel beyond the limits of the individual schemes, and allows determination of the mean photon energy. Medical applications such as X-ray computed tomography can benefit from this additional spectral information through improved contrast. The pixel disclosed contains three basic elements: a photon counter, an integrator and a special feedback circuit which provides both signal shaping for the photon counter and signal replication for the integrator.